By the development of an information society, a communication capacity has been increased remarkably. In many examples, a band which has not been used yet is utilized for a transmitting medium in order to increase the communication capacity. In radio communication, although a specific band is basically utilized with a limitation to specific uses as provided by law, some bands are opened to a plurality of uses. For UWB (Ultra Wide Band) system, a very broad band can be used with a limitation of a low power short distance communication and is opened to overlap with other uses. In this case, there is a possibility that one of radios might be an interference wave with the other radio. For this reason, an interference avoiding technique referred to as DAA (Detect And Avoid) has been obliged to the UWB at a part of frequencies. More specifically, in the case in which a UWB terminal performs a communication at a certain frequency, a radio wave must stop transmission at the frequency upon receipt of the radio wave of a system having a preferential transmission right at the frequency.
With an enlargement of a concept of the DAA, there is reached a radio system in which a radio wave is generated at a frequency when it is confirmed that the frequency is not utilized, and a communication is stopped when a system having a priority starts a transmission at the frequency. This is referred to as a cognitive radio. In the cognitive radio, the system utilizes frequencies excluding the frequency which is being utilized by a system having the priority. Therefore, the frequencies which can be utilized are brought into a moth-eaten state. A range which can be contiguously utilized is varied depending on the frequency.
In order to maintain a necessary bit rate for a high speed communication, it is necessary to utilize a wide frequency range in total to some degree. For this reason, the frequency in the moth-eaten state composed of many bands is to be utilized allowing discontinuity. In this case, there is a higher possibility that a method of digitally synthesizing, separating, modulating and demodulating the bands by using one analog processing unit and A/D (analog-digital) and D/A (digital-analog) converters might be taken in place of modulating and demodulating those bands with different systems, respectively.
When a certain frequency is being utilized, it is necessary to periodically confirm that a terminal of a system having a priority have not started a transmission at the frequency. It is hard to perform the confirmation while performing a communication at the frequency. For this reason, the utilization of the frequency is often stopped properly to detect other radio waves through the execution of carrier sensing in a state in which the cognitive system is not generating the radio wave. At this time, it is also necessary to detect a radio wave having a power which is as low as possible in order to avoid a hidden terminal problem (a problem in the case in which a transmission is performed on the assumption that other systems are distant because transmitters of the other systems are disposed distantly, and receivers of the other systems are near to a cognitive terminal and cause a interference) as far as possible.
In the case in which the discontiguous frequency bands are received in a lump at the same time as described above, transmitted radio waves from other systems are interposed between the bands utilized by the cognitive terminal. Even if the transmission of the cognitive system is stopped to detect other systems' transmission, the radio waves of other systems interposed between the bands are being continuously transmitted. A cognitive terminal does not want to confirm the contents of the radio waves of the other systems which are continuously transmitted but the fact that the frequency which is being utilized by the cognitive terminal interposed therebetween is used by only a cognitive system and is empty. In the case in which broad bands are received in a lump at the same time, the radio waves transmitted by the other systems are also received together until an A/D conversion and filtering in a digital circuit are performed. In order to perform the carrier sensing to confirm that the other systems do not transmit the radio waves at that time, it is necessary to detect any small signal which is near to a threshold. In some cases in which a large number of other systems utilize frequency bands interposed between the bands utilized by the cognitive terminal. In other cases, a system originally having high PAPR (Peak-to-Average Power Ratio) and a high receiving power is included. In these cases, a very high peak is suddenly generated even if a total average power of the systems is not very high.
When a high PAPR signal is generated, a clipping is caused in an A/D converter if a signal is not suppressed before the A/D converter in order to prevent the A/D converter from being saturated. When the clipping is caused, a large amount of spurious noise (a noise component) referred to as a clipping noise is generated. In many cases, the clipping generates the spurious noise almost uniformly over frequencies. The amount of the spurious noise depends on amplitude and number of times which are subjected to the clipping and a period subjected to the clipping at a time. In many cases in which the signal is received in such a large dynamic range that a thermal noise can be observed, the amount of the spurious noise exceeds the thermal noise. In these cases, even if there are the radio waves of the other systems which have a near level to the thermal noise, there is a possibility that they might be buried in the spurious noise due to the clipping and cannot be detected.
As a simple method of preventing the spurious noise by the clipping, a large margin for an input of the A/D converter is taken to cause a sudden peak to enter a range of an A/D conversion even if the peak is generated. In the method, however, a quantization noise is relatively increased. Depending on the amount of the margin, it is also impossible to detect a small signal if the quantization noise is larger than the thermal noise. It is preferable to increase the number of bits of the A/D converter in order to reduce the quantization noise. However, the A/D converter is generally a component having larger power consumption in receivers. In particular, the number of quantization bits greatly influences the consumed power. In respect of a terminal design, accordingly, it is not preferable that the number of the quantization bits should be increased for a peak which is sometimes generated suddenly.
The generation of the spurious noise due to the clipping is often a problem in a D/A converter of a transmitter. For example, there has been known a problem in that the same situation is generated when a digital signal having a sudden peak is to be converted into an analog signal, and a signal output from a transmitter contains a large amount of unnecessary radiation due to the clipping noise. For example, see JP-A-2004-128923.
In the Background section of the document JP-A-2004-128923, a method of previously clipping a signal on a level corresponding to a clipping level of a D/A converter before a D/A conversion, filtering spurious noise which is generated by the pre-clipping and performing the D/A conversion has been described as a technique for preventing the generated spurious noise. The method has a problem in that a peak which should have been clipped is reproduced by the filtering.
The document JP-A-2004-128923 disclosures a technique in which a waveform is smoothed by using a moving average in place of the pre-clipping, a signal exceeding the clipping level is caused to have a gain on a logarithmic amplifier basis, and the peak is suppressed, and at the same time, an extreme clipping is eliminated. Referring to a method of thus taking a countermeasure against the spurious noise in a digital circuit, it is premised that a signal before clipping has been known to the digital circuit.
The digital circuit cannot know a signal before the A/D conversion. Accordingly, a method of suppressing the generation of the spurious noise by the D/A converter which needs information about a signal before clipping cannot be simply applied to the A/D converter.
In the method of clipping and filtering, a signal having a low power is also removed by filtering at the same time. Therefore, the method is unsuitable for the DAA which is required to detect signals transmitted by the other systems in a frequency in which the spurious noise is generated.